Alkanes, addition radical coupling

Group VIA -Cr, Mo, W. The stable, water-soluble metal alkyl [(H20)5-Cr( y -<7-CH2C6H5)] is obtained by one-electron oxidative additions to [Cr-(H20)J with reactive alkyl halides. The rate-determining, halogen-transfer step generates an alkyl free radical, which then rapidly reacts with a second molecule of Cr(ll) ion or undergoes radical coupling and reduction to alkane ... 

Once formed, the radical intermediate (R ) can couple to afford a dimer (R2), can disproportionate to give an alkane (RH) and an olefin (R(—H)), or can accept a hydrogen atom from a donor (such as the solvent, SH) to give an alkane. A carbanion (R ) can be protonated by the solvent (or a deliberately added acid, HB) to yield an alkane. In addition, RX can undergo E2 and Sn2 reactions with B , and R can attack RX to form a dimer. 

The atmosphere above the ocean contains C9-C28 n-alkanes in addition to low-weight hydrocarbons. Eichmann et al. (1980) have estimated that the ocean might emit 26 Tg of higher alkanes each year. The value is based on the measured total atmospheric concentration for C9-C28 n-alkanes of 300-400 ng/m3 over the Indian and the North Atlantic Oceans, coupled with an assumed residence time of the alkanes of 5 days resulting from their reactions with OH radicals. From measurements at Enewetak Atoll (Marshall Islands), however, Duce et al. (1983) reported atmospheric concentrations of C,2-C28 alkanes that were an order of magnitude lower than those found by Eichmann et al. (1980). It is questionable, therefore, whether their data can be extrapolated to the whole ocean area, so that the emission rate derived must be regarded an upper limit. 

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

As noted above, irradiation of 2-norbornene (14) in pentane afforded, in addition to the carbene-derived products 17 and 18, a mixture of alkanes 20-22, which appear to be derived from the radical 19, formed by hydrogen atom abstraction from the solvent. Analogous behavior was observed for 1-octene (10), which afforded the reduction product octane (13), and cyclopentene (32), which afforded the radical-derived products 34 and 35. In each case, a mixture of branched decanes arising from coupling of solvent-derived pentyl radicals was also obtained. Hydrogen-atom abstraction has also been attributed to the nn excited state. ... 

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